Measurement circuit for load cell

ABSTRACT

The strain gauge bridge circuit located in .[.the.]. .Iadd.a .Iaddend.load cell .[.of a load cell mass comparator.]. is modified to provide temperature stability by coupling a remotely located temperature compensating circuit between the two normally connected output ends of strain gauges in adjacent arms of the bridge. The compensating circuit is comprised of a pair of series connected low noise, drift-free precision resistors of relatively low resistance value compared to the resistance of the strain gauges. The precision resistors are shunted by relatively high valued potentiometers which operate to balance the bridge. One potentiometer additionally includes a series connected high value precision resistor for providing fine balance while the other potentiometer is used for coarse balance.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to U.S. Ser. No. 06/587,532, entitled, "LoadCell Mass Comparator", filed in the names of Thomas F. Scrivener andRandall M. Schoonover, on Mar. 8, 1984, .[.and.]. is assigned to theassignee of this invention.[...]. .Iadd., and now U.S. Pat. No.4,523,653..Iaddend.

BACKGROUND OF THE INVENTION

To overcome the disadvantages experienced in the calibration of massstandards employing conventional mechanical balances, it has beenproposed to use a strain gauge load cell which produces an electricsignal in proportion to the force exerted on the cell. Typically, aplurality of strain gauges are located in the cell and are internallyconnected in the form of a bridge circuit, the output of which isconnected to electronic measuring and recording equipment. Where theelectronic measuring circuit is comprised of a low noise, high stabilityelectronic circuit, wide fluctuations at the output of the bridgecircuit due to temperature drift is not only undesirable, butintolerable where difference measurements are made between a standardweight and a test weight.

Accordingly, the present invention is directed to .[.an.]. .Iadd.a.Iaddend..[.improved.]. .Iadd.temperature stabilized .Iaddend.bridgecircuit formed by the strain gauges located in .[.the.]. .Iadd.a.Iaddend.load cell .[.of.]. .Iadd.such as, but not limited to .Iaddend.aload cell mass comparator. The bridge circuit is modified for use as adifference transducer by coupling a temperature compensating circuitbetween ends of strain gauges in adjacent arms of the bridge. Thecompensating circuit is located remotely from the load cell containingthe strain gauges and is comprised of a pair of relatively low valuedseries connected low noise, drift free precision resistors, which arerespectively shunted by relatively high valued resistive potentiometerswhich are used for balancing the bridge. One potentiometer additionallyincludes a series connected relatively high valued low noise, drift freeprecision resistor for providing a means for providing fine balance ofthe bridge, while the other potentiometer is used for providing coarsebalance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a mechanical schematic diagram of a load cell mass comparator.[.utilized in connection with the present invention.].;

FIG. 2 is an electrical schematic diagram of a typical prior art bridgecircuit of strain gauges located in the load cell mass comparator shownin FIG. 1; and

FIG. 3 is an electrical schematic diagram of the preferred embodiment ofthe subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and more particularly to FIG. 1, there isshown a load cell mass comparator of the type shown and described in theabove referenced .[.related application, U.S. Ser. No. 587,532 filedMar. 8, 1984, now.]. U.S. Pat. No. 4,523,653. As shown in FIG. 1,reference numeral 1 denotes a conventional load cell which may be, forexample, a type manufactured by Revere Corporation of America, and whichincludes a plurality of strain gauges, not shown, internally wiredtogether to form a Wheatstone bridge such as shown in FIG. 2 and whichadditionally includes an output cable 2 for being coupled to anexternally located supply and measuring circuit, not shown.

The load cell 1 is connected at its upper end to a floating plate 3through a self aligning coupling such as a universal joint assembly 4.The floating plate 3 is slidably mounted on a plurality of guide rods 5extending between a fixed upper plate 6 and a fixed lower plate 7. Aspring and shock absorber assembly shown schematically by referencenumeral 8 is mounted between the floating plate 3 and the upper fixedplate 6. The lower end of the load cell 1 is connected to the mass to becalibrated, not shown, through a second self-aligning coupling 9, athrust bearing 10, and a load stop bearing 11. The mass to be calibratedis connected to an eyelet 12 provided on the lower end of a rod 13connected to the thrust bearing 10 and the load stop bearing 11. Anothereyelet 14 is secured to the upper fixed plate 6 through a hydrauliccylinder 15 so that the entire assembly may be suspended from a suitablesupport and the comparator can be loaded and unloaded by actuation ofthe hydraulic cylinder.

As force is applied to the load cell 1, minute deflections are imposedon the internally located strain gauges resulting in changes in thecross section thereof. As is well known, the strain gauges areelectrical components whose resistance changes upon the application ofexternal force. Accordingly, the strain gauges in the load cell 1 arerepresented as resistors R₁, R₂, R₃ and R₄ in both FIGS. 2 and 3 and areconnected in a bridge circuit configuration. With a fixed excitationvoltage applied across the bridge, for example, at terminals 16 and 17(FIG. 2) and which are connected to a first pair of mutually opposingcircuit junctions 18 and 19, the bridge becomes unbalanced and an outputvoltage is generated across a second pair of mutually opposing junctions20 and 21. The output voltage is then coupled to the output terminals 22and 23 and is proportional to the applied load, which when coupled tothe measuring circuit, not shown, can be appropriately amplified,displayed, printed or otherwise interfaced to a fully automated controlsystem.

In order to compensate for temperature changes experienced by the loadcell 1 and the strain gauges included therein, reference is now made toFIG. 3 which discloses the preferred embodiment of the invention whichcomprises a modification to the bridge circuit shown in FIG. 2.Accordingly, an integrated strain gauge supply, measuring and recordinginstrument 24 is shown located remotely from the load cell masscomparator containing the load cell 1 by a distance d which may be, forexample, 25 feet or more. The inventive concept is directed to anexternal resistive type compensation circuit connected into outputjunction 21 (FIG. 2) intermediate the output side of the strain gaugesR₂ and R₃, which form two adjacent forms of the bridge. The compensatingcircuit 25, moreover, is preferably located remote from the strain gaugebridge, such as being located in close proximity to or incorporated withthe measuring instrument 24 and thus being separated from the bridge bythe distance d.

The temperature compensation circuit 25 is comprised of a pair of seriesconnected precision resistors having fixed values of relatively lowresistance in comparison to the resistance values of the strain gaugeelements R₁, R₂, R₃ and R₄ but exhibiting low noise and low temperaturedrift characteristics. The precision resistors 26 and 27 are shown inFIG. 3 coupled between the terminals 21_(a) and 21_(b) by means ofelectrical connecting leads 28 and 29 which span the length d.

The addition of the two precision resistor elements 26 and 27 permit twoadditional points in the bridge to be accessed electrically, namely thecircuit junctions 30, 31 and 32 instead of the single junction 21, asshown in FIG. 2. The compensating circuit 25 in addition to two seriesresistors 26 and 27, however, additionally includes circuit means whichpermits the bridge to be balanced during operation, i.e., when it isunder strain because of an applied load. The balancing means comprises apair of relatively high valued variable resistances in the form ofpotentiometers 31 and 34, respectively shunting the low valued precisionresistors 26 and 27. One potentiometer, specifically potentiometer 33additionally includes a series connected high valued precision resistorof fixed value so that the potentiometer 33 can be utilized as a meansfor fine balance, whereas the single potentiometer 34 coupled across theresistor 27 can be utilized to provide a coarse balance. The slider ofthe potentiometer 34, moreover, is coupled to an output lead 36 whichcouples to terminal 22 which is shown in FIG. 3 located on the measuringinstrument 24 along with terminal 23 which connects back to the otheroutput junction 20 of the bridge by circuit lead 37.

The values of the two potentiometers 33 and 34 as well as the thirdfixed resistor 35 are selected to be of a much greater resistance valuethan the pair of fixed resistors 26 and 27 so that the operation of thepotentiometers 33 and 34 will not degrade the performance of the bridgeand as a result can be nulled at any load within its capacity sincethere will always be a point between terminals 21_(a) and 21_(b) whichwill be at the same potential as the opposing output circuit junction20.

In operation, the null appearing at output terminals 22 and 23 is firstdetermined by weighing a standard mass. Then the mass to be calibratedis weighed. The strain gauge bridge which was previously nulled by usingthe standard mass will then indicate the difference between the weightto be calibrated and the standard weight. The temperature stability ofthe configuration as shown in FIG. 3 is significantly improved over thestandard or conventional prior art method as shown in FIG. 2 because theadditional precision resistors have a value which is small in relationto the resistance of the strain gauge elements R₁, R₂, R₃ and R₄ and areconnected such that the symmetry of the bridge is preserved.

In the event that a fine adjustment of bridge balance is not required,the fine balance potentiometer 33 and the series connected fixedprecision resistor 35 can be eliminated, leaving only the pair of lowvalue precision resistors 26 and 27 connected between the terminals21_(a) and 21_(b) and with but a single potentiometer such aspotentiometer 34 shunting one of the precision resistors.

It is to be understood that the embodiment of the invention herewithshown and described is to be taken as a preferred example and beingshown for purposes of illustration and not limitation. Accordingly,various changes, modifications, and alterations may be resorted towithout departing from the spirit of the invention or scope of thesubjoined claims.

I claim:
 1. A temperature compensated load sensing circuit for a loadcell mass comparator including a load cell containing a plurality ofstrain gauges connected to form arms of a bridge circuit connected to ameasuring circuit, comprising the combination of:a pair of seriesconnected resistance means coupled between the output side of the straingauges in a first of two adjacent arms of said bridge circuit, saidresistance means being of a relatively low resistance value compared tothe resistance value of said strain gauges; potentiometer means shuntedacross at least one of said pair of series connected resistance means,said potentiometer means having a relatively large resistance valuecompared to the resistance value of said resistance means and furtherincluding an adjustable voltage output terminal; and means connectingsaid output terminal of said potentiometer means and the output side ofthe strain gauges in the second of two adjacent arms of said bridgecircuit to said measuring circuit.
 2. The load sensing circuit accordingto claim 1 wherein said pair of series connected resistance meanscomprises a first and second precision resistor.
 3. The load sensingcircuit according to claim 2 wherein said first and second precisionresistors further exhibit a low noise and low temperature driftoperating characteristic.
 4. The load sensing circuit according to claim3 wherein said first and second precision resistors and saidpotentiometer means are located remotely from said load cell.
 5. Theload sensing circuit according to claim 1 and including additional.Iadd.bridge balancing .Iaddend.potentiometer means shunted across theother of said pair of series connected resistance means, said additionalpotentiometer means also having a relatively large resistance valuecompared to the resistance value of said resistance means.
 6. The loadsensing circuit according to claim 5 and further including resistancemeans coupled in series with said additional potentiometer means wherebythe first recited said potentiometer means is operable to provide acoarse balance of said bridge and said additional potentiometer means isoperable to provide a fine balance of said bridge.
 7. The load sensingcircuit according to claim 6 wherein said pair of series connectedresistance means and said resistance means connected in series with saidother potentiometer means are comprised of precision resistors.
 8. Theload sensing circuit according to claim 7 wherein said precisionresistors are comprised of resistors having a low noise and lowtemperature drift characteristic.
 9. The load sensing circuit accordingto claim 8 wherein said precision resistors and both said potentiometermeans are located remoately from said load cell.
 10. The load sensingcircuit according to claim 9 wherein said remotely located precisionresistors and potentiometer means are located in close proximity to saidmeasuring circuit.
 11. The load sensing circuit according to claim 9wherein said remotely located precision resistors and potentiometermeans are integrated with said measuring circuit.
 12. The load sensingcircuit according to claim 9 wherein said precision resistors are fixedvalue resistors. .Iadd.
 13. A circuit for providing temperaturestabilization of a load cell containing a plurality of strain gaugesconnected to form arms of a bridge circuit connected to a measuringcircuit, comprising the combination of:a pair of series connectedimpedances coupled between the output side of the strain gauges in afirst of two adjacent arms of said bridge circuit, said pair ofimpedances being of a relatively low impedance value compared to theimpedance value of said strain gauges; variable impedance means forbalancing said bridge circuit shunted across at least one of said pairof series connected impedances, said variable impedance means having arelatively large impedance value compared to the impedance value of saidpair of impedances and further including an adjustable voltage outputterminal; and means connecting said output terminal of said variableimpedance means and the output side of the strain gauges in the secondof two adjacent arms of said bridge circuit to said measuringcircuit..Iaddend. .Iadd.14. The circuit according to claim 13 whereinsaid pair of series connected impedances comprises a first and secondprecision resistor..Iaddend. .Iadd.15. The circuit according to claim 14wherein said first and second precision resistors further exhibit a lownoise and low temperature drift operating characteristic..Iaddend..Iadd.16. The circuit according to claim 15 wherein said variableimpedance means comprises potentiometer means and wherein said first andsecond precision resistors and said potentiometer means are locatedremotely from said load cell..Iaddend. .Iadd.17. The circuit accordingto claim 13 and including additional variable impedance means forfurther balancing said bridge circuit shunted across the other of saidpair of series connected impedances, said additional variable impedancemeans also having a relatively large impedance value compared to theimpedance value of said impedances..Iaddend. .Iadd.18. The load sensingcircuit according to claim 17 and further including impedance meanscoupled in series with said additional variable impedance means wherebythe first recited said variable impedance means is operable to provide acoarse balance of said bridge and said additional variable impedancemeans is operable to provide a fine balance of said bridge..Iaddend..Iadd.19. The circuit according to claim 18 wherein both said variableimpedance means are comprised of potentiometer means and wherein saidpair of series connected impedances and said impedance means connectedin series with said other variable impedance means are comprised ofprecision resistors..Iaddend. .Iadd.20. The circuit according to claim19 wherein said precision resistors are comprised of resistors having alow noise and low temperature drift characteristic..Iaddend. .Iadd.21.The circuit according to claim 20 wherein said precision resistors andboth said potentiometer means are located remotely from said loadcell..Iaddend. .Iadd.22. The circuit according to claim 21 wherein saidremotely located precision resistors and potentiometer means are locatedin close proximity to said measuring circuit..Iaddend. .Iadd.23. Thecircuit according to claim 21 wherein said remotely located precisionresistors and potentiometer means are integrated with said measuringcircuit..Iaddend. .Iadd.24. A circuit for providing temperaturestabilization of a bridge circuit including a plurality of impedancemeans connected to form arms of a bridge circuit connected to ameasuring circuit, comprising the combination of:a pair of seriesconnected impedances coupled between the output side of the impedancemeans in a first of two adjacent arms of said bridge circuit, said pairof impedances being of a relatively low impedance value compared to theimpedance value of said impedance means; variable impedance means forbalancing said bridge circuit shunted across at least one of said pairof series connected impedances, said variable impedance means having arelatively large impedance value compared to the impedance value of saidpair of impedances and further including an adjustable voltage outputterminal; and means connecting said output terminal of said variableimpedance means and the output side of the impedance means in the secondof two adjacent arms of said bridge circuit to said measuringcircuit..Iaddend. .Iadd.25. The circuit according to claim 24 whereinsaid bridge circuit comprises a load sensing circuit..Iaddend. .Iadd.26.The circuit according to claim 24 wherein said bridge circuit comprisesa load sensing circuit included in a load cell..Iaddend. .Iadd.27. Thecircuit according to claim 24 wherein said plurality of impedance meanscomprises a plurality of strain gauges included in a load cell..Iaddend.